Electronic devices, such as integrated circuit dice, are packaged using a variety of materials. For example, plastics, ceramics, and glasses are used as substrates and die carriers, while adhesives and polymers are used to attach dice to substrates or other die carriers. Each of the materials used to package electronic devices has a coefficient of thermal expansion that defines a rate of expansion for the material as the temperature of the material changes.
FIG. 1A is a cross-sectional view of a prior art electronic package 101 including an adhesive having a low Young's modulus. The electronic package 101 includes a substrate 103, one or more solder balls 105, a die 107, a die attach material 109, a molding compound 111, and a board 112. In this exemplary package, substrate 103 is fabricated from a ceramic, solder balls 105 are fabricated from a conductor, such as a lead-tin alloy, die 107 is fabricated from a semiconductor, such as silicon, germanium, or gallium arsenide, die attachment material 109 is fabricated from a compliant material, such as an adhesive having a low Young's modulus, molding compound 111 is formed from an epoxide, and board 112 is fabricated from a glass-epoxide. The bond wires 114 electrically couple the die 107 to a bottom surface 116 of the substrate 103.
FIG. 1B is a cross-sectional view of an alternative prior art electronic package 125 including an adhesive having a low Young's modulus. The electronic package 125 includes a substrate 103, one or more solder balls 105, a die 107, a die attachment material 109, a molding compound 111, and a board 112. In this exemplary package, substrate 103 is fabricated from a ceramic, solder balls 105 are fabricated from a conductor, such as a lead-tin alloy, die 107 is fabricated from a semiconductor, such as silicon, germanium, or gallium arsenide, die attachment material 109 is fabricated from a compliant material, such as an adhesive having a low Young's modulus, molding compound 111 is formed from an epoxide, and board 112 is fabricated from a glass-epoxide. The bond wires 114 electrically couple the die 107 to a top surface 127 of the substrate 103.
In the manufacture and assembly of electronic device packages, the devices and packages are often subject to temperature changes. For example, during the manufacture of electronic packages 101 and 125 (shown in FIGS. 1A and 1B, respectively), after substrate 103 is positioned on solder balls 105 above board 112, the temperature of solder balls 105 is increased, which causes solder balls 105 to flow and form an electrical connection between substrate 103 and board 112. The temperature change that causes the reflow of solder balls 105 also causes a temperature change in die 107, substrate 103, and die attachment material 109.
Temperature changes can cause mechanical stresses in packages 101 and 125. Thermal mechanical stress is caused in packages 101 and 125 by a change in temperature and a mismatch between the coefficients of thermal expansion of die 107, substrate 103, and die attachment material 109. Moisture induced stress is caused by the vaporization, at high temperatures, of moisture retained in substrate 103 or die attachment material 109. Either thermal mechanical stress or moisture induced stress can cause mechanical tolerances in package 101 to be exceeded, which results in package failure. Referring to FIGS. 1A and 1B, package failures include package cracking, as shown at cracks 118 and 120, delamination of die attachment material, as shown at delamination point 122, deformation of die attachment material, as shown at deformation point 124, and other failures. These failures can result in a reduction in device reliability and may cause total device failure. U.S. Pat. No. 5,679,977 and U.S. Pat. No. 5,852,326 teach that attaching a die to a substrate using a material having a low Young's modulus produces fewer package failures than attaching a die to a substrate using a material having a high Young's modulus, and that improved packaging reliability is obtained by selecting a die attachment material having a very low Young's modulus. In electronic package 125 (shown in FIG. 1B) defects, such as deformation 129, occur in the die attachment material 109, and deformation 129 of the die attachment material 109 can result in dislocation of the die 107 and destruction of the bond wires 114.
Electronic packages, such as electronic package 101 and electronic package 125, are tested using a temperature cycle test. In a temperature cycle test, an electronic package, such as electronic package 101 or electronic package 125, is repeatedly heated and cooled. In one form of the temperature cycle test, electronic packages 101 and 125 are repeatedly heated and cooled between minus 65 degrees Centigrade and 150 degrees Centigrade. Often this cyclic testing results in “cyclic strain” failures. These failures include separation of die attachment material 109 from die 103 and substrate 107 and deformation of die attachment material 109. Electronic packages that fail a “cyclic strain” test also often fail prematurely in the field.
For these and other reasons there is a need for the present invention.